Lanthanide Bimetallic Helicates forin VitroImaging and Sensing

Abstract

As the need for targeting luminescent biolabels increases, for mapping selected analytes, imaging of cells and organs, and tracking in cellulo processes, lanthanide bimetallic helicates are emerging as versatile bioprobes. The wrapping of three ligand strands around two metallic centers by self-assembly affords robust molecular edifices with tunable chemical and photophysical properties. In addition, heterometallic helical chelates can be assembled leading to bioprobes with inherent chiral properties. In this paper, we review the literature demonstrating that neutral [Ln(2)(L(CX))(3)] (x=1-3) helicates represent a viable alternative to existing chelating agents for bio-analyses, while featuring specific enhanced properties. These bimetallic chelates self-assemble in water, and at physiological pH the 2:3 (Ln:L(CX)) complex is by far the dominant species, conditional stability constants logbeta(23) being in the range 23-30. The metal ions are 9-coordinate and lie in sites with slightly distorted D(3) symmetry. Efficient protection from water interaction by the tightly wrapped ligand strands results in sizeable photophysical properties, with quantum yields up to 24% for Eu(III) and 11% for Tb(III), while the luminescence of several other visible and/or near-infrared emitting Ln(III) ions is also sensitized. Noncytotoxicity for all the helicates is established for several living cell lines including HeLa, HaCat, MCF-7, 5D10, and Jurkat. We present new data pertaining to the live cell imaging ability of [Eu(2)(L(C1))(3)] and compare the three systems with x=1-3 with respect to thermodynamic stability, photophysics, cell-permeation ability, and targeting capability for sensing in cellulo processes. Prospects of derivatization for characterizing specific biological interactions are discussed.

Basics of Lanthanide Photophysics

“…The bracketed matrix elements are tabulated and the radiative lifetime can, therefore, be extracted from the spectral intensity, that is from (12), (20), and (21). Except in few cases, this calculation is not trivial and large errors may occur, including those pertaining to the hypotheses made within JO theory.…”

“…Therefore, it is advantageous to resort to polydentate ligands for building a coordination environment around Ln III ions. Macrocyclic complexes based on the cyclen framework [22] or on cryptands [23] are also proved to be quite adequate, as well as self-assembled mono-and bi-nuclear triple helical edifices [21].…”

“…Moreover, the energy of this transition depends on the nephelauxetic effect d i generated by coordinated atoms and ions; at 298 K: with C CN being a constant depending on the coordination number and n i the number of coordinating groups with nephelauxetic effect d i . The latter are tabulated for the most common ligands [32] and some predictions are rather accurate [21], which allows one to check the composition of the inner coordination sphere. When several metal ion sites are present in a compound, population analysis can be carried out in two main ways.…”

Multiphoton-Excited Luminescent Lanthanide Bioprobes: Two- and Three-Photon Cross Sections of Dipicolinate Derivatives and Binuclear Helicates

“…6,7 In addition, recent developments in the design of lanthanide luminescent bioprobes (LLBs) allow us to optimize other important properties like water solubility, cell permeability, noncytotoxicity, kinetic inertness, or thermodynamic stability at physiological pH, as well as to derivatize LLBs with functional groups for further coupling with biological molecules and specific organelles. 6,[8][9][10][11] However, despite that the feasibility of multiphoton scanning microscopy for imaging of living cells was demonstrated with compounds possessing low MPA cross sections, 12,13 the number of investigations devoted to MPA LLBs remains scarce. 7 The first two-photon microscopy images were obtained for crystals of the Tb III lysozyme derivative of tris(dipicolinate).…”

“…16 In view of the intrinsic advantages presented by double lanthanide tags 17 our laboratory has recently focused on the development of LLBs based on binuclear helicates. 8 In this work we aim at enlarging the potential uses of these compounds by shifting the excitation wavelength into the NIR range to which biomolecules are transparent. Since multiphoton excitation appears to be a convenient solution, luminescent properties of tris(dipicolinate) derivatives and binuclear helicates are investigated under 800 nm excitation from an amplified femtosecond Ti:sapphire laser system and compared to those of the parent tris(dipicolinates).…”